With ever increasing applications and utilization of electrical and electronic control systems composed of sensors, control electronics, and actuators, there are concurrent and ever increasing technical difficulties pertaining to reliable implementation thereof. Additionally, urgency in rushing product development to facilitate getting devices to market, ongoing incremental technical developments, and desires to include upgraded functional features increasingly cause product obsolescence prior to end of life failure.
Numerous new sensors are constantly being developed, many of which push the limits of physical properties and principles. Some sensors transduce very small changes in physical signals with high resolution and accuracy into low voltage, low current, high impedance analog electrical signals. Modern control system electronics typically tend toward various features including: higher clock frequencies; higher switching slew rates; lower voltages; lower and higher currents; widely distributed controls; direct digital control of power electronics; wider environmental operating conditions; and wire harnesses containing mixed conductor types of analog, digital, and power conductors.
Improvements in modern electronic control systems, power electronic components, and high power electrical actuators now enable sophisticated control of electrical transducing actuators to perform tasks previously practical only by other non-electrical methods. High power actuators, now in typical use, require high levels of voltage and/or current. One result of these technological advancements in sensing, electronics, and actuators is numerous new opportunities for electrical noise caused by various transients and frequencies of voltages and currents of a system to interfere with another system or even with itself. Witness the numerous standards issued by major technical societies and industrial nations pertaining to allowable levels of wideband radio frequency interference (RFI) emissions and susceptibility via conduction and field transmission. An additional result is obsolescence (sometimes planned) of existing electrical and electronic devices by new, improved devices offering additional features.
Interference noise from zero Hz through RF is transmitted via four major methods consisting of: Electric field (e-field), magnetic field (m-field), electromagnetic (EM) field, and conducted. The fields are transmitted via contactless transmission.
The space surrounding a source of radiation can be broken into two regions, near or induction field and far or radiation field. Near field is usually defined as the distance closer than the wavelength divided by 2.pi.. Far field is greater than this distance. The ratio of E field to H field is the wave impedance. In the far field the E/H ratio equals the characteristic impedance of the medium. (377 ohm for free space). In the near field, the ratio is determined by the characteristics of the source and the distance from the source to where the field is observed. If the source has high current and low voltage (E/H&lt;377), the near field is predominantly magnetic, high voltage low current is E/H&gt;377, the near field is predominantly electric. Usually when individual electric and magnetic field are discussed, they are assumed to be in the near field. Also in the near field region, fields are strongly dependent on distance since the predominant of H and E field fall of as 1/D 3 and the lesser of H and E field fall off as 1/D 2 so that the characteristic wave impedance approaches that of the medium as it leaves the source and asymptotically becomes that of the medium at far field distances beyond .lambda./2.pi..
Conducted transmission is essentially electrical energy passing via direct electrical connection. High voltages produce strong e-fields. High currents produce strong m-fields. High voltages and/or high electrical currents oscillating at high frequencies or high acceleration of charges produce strong EM fields. EM fields from the very low frequency (long wavelengths on scale of kilometers and longer) to extremely high frequency (short wavelengths on scale of molecules and shorter) are generally composed of orthogonal coupled e-fields with m-fields which emanate at the speed of radio waves in that medium and which may also have such characteristics as being polarized, rotating, coherent, or incoherent. EM antenna characteristics of a device at a given frequency relate to geometry, conductivity, and dielectric coupling with its environment. A device which is a good field emitting or transmitting antenna with high emissivity characteristics is generally also a good field receiving antenna with associated high susceptibility characteristics.
Undesired interference is called noise. Not all interference is man-made. Natural phenomena which produce high (RF) and/or low frequency interference include: solar wind (atmospheric ionization), electrical storms (lightning), static electricity discharge (sparks), geomagnetic storms (ground currents), and more. Many sources of interference are man-made devices operating as designed and in other cases, not operating as intended. Newer electronic devices and systems tend to be more sensitive and thus can be more affected by various natural and man-made sources of interference. It is possible that devices engineered and manufactured to older specifications can produce interference which is not readily apparent to the casual observer but which can have catastrophic effect upon susceptible devices.
Electromagnetic compatibility (EMC) is here understood to relate to components, devices, and systems compatible with respect to noise transmitted and/or received via conduction and/or field transmission, especially in the RF spectrum. Typical specifications relate quantities and durations of anomalous system operation allowed versus quantity of interference of various types, examples of which a system output may have are: no effect, temporary effect on performance or accuracy, permanent effect on performance or accuracy, and functional death of the system. Critical safety related applications mandate a very high reliability and tolerance to all types of interference and environmental conditions. Modern application customers expect and demand reliable operation from all components and systems under all anticipated operating conditions. The need increasingly exists for systems and component devices to reduce both interference emitted and susceptibility to external noise emissions received.
One significant source of electrical noise is motors for which currents and voltages are switched or commutated. This includes direct current (DC) motors and also some types of alternating current (AC) motors such as AC commutator motors, universal motors, and motors for which the alternating currents are generated by switching inverter circuits. Motors can be particularly noisy under certain transient conditions, for example, high startup current, and overspeed of an under-loaded series DC motor. For many of the existing motor designs in production and common use, it is desired simply to attenuate the RFI noise which they produce to maintain compliance with newer EMC regulations. It is common knowledge that various types of motor operation emit noise that can be detected by radio or television receivers.
Modern motor applications can include electrical analog and digital signal-carrying wires to and from the motor along with the power supply wires.
These signals are typically used with closed loop feedback control and safety systems for monitoring and control of such motor parameters as position (rotary or linear), velocity, acceleration, stator and armature resolver and encoder voltage waveforms, stator and armature current and voltage waveforms, rotor torque (also the linear motor analog of force), stator and armature temperatures, and the like.
In various applications, signal processing circuitry (analog and/or digital) is incorporated: (1) within the motor housing; (2) attached to the motor, near but separate from the motor; or (3) remote from the motor. This control circuitry and thus the system functional operation can potentially be detrimentally affected by noise generated within the motor itself and/or by outside sources and transmitted by: motor wiring, magnetic fields, electric fields, or via EM fields. New demand and consideration are being given for use of multiple critical and sensitive low-level and/or high speed analog and/or digital motor sensing and control circuits which are potentially sensitive to RFI conduction, emissivity and susceptibility.
U.S. Pat. No. 4,698,605 discloses an electronic filter component for attenuation of high frequency conducted electrical current noise of a single conductor (and thus its associated radiated noise), being comprised simply of a cylinder of semiconductor ferrite having also some capacitive properties imparted by virtue of metallizing the outer circumference and optionally the inner bore such that with or without ohmic contact of the conductor with the inner metallized bore the component will have properties of distributed inductance and capacitance to perform in the functional capacity as a low pass common mode electronic current filter.
U.S. Pat. No. 4,992,060 discloses an electronic filter device having a standard ferrite component with no metallization. Inner metallization is functionally supplied by a metal shield around the wire bundle. Outer metallization is functionally supplied and held proximal by the inside of the connector shell, shaped to be captive and protected within the connector housing, and optionally being axially split to fit within the two halves of the connector. The component is integral within at least one end of a shielded cable connector for attenuation of high frequency conducted electrical current noise primarily of the shield of a shielded conductor bundle (and thus its associated radiated noise) such that the ferrite with metal contact on its inner and outer cylindrical surfaces also has some capacitive properties imparted by virtue of the metallic contact with the outer circumference and the inner bore will have properties of distributed inductance and capacitance to perform in the functional capacity as a low pass common mode electronic current filter.
U.S. Pat. No. 5,500,629 discloses an electronic filter component for attenuation of high frequency conducted electrical current noise (and thus its associated radiated noise) of a single conductor (and/or with multiple use for multiple conductors), being comprised simply of a sandwich-like shape of special types of engineered non-linear ferrite having inductive, resistive, and capacitive properties in bulk and also some capacitive properties imparted by virtue of metallizing electrical contact areas such that the component will have properties of transient clipping with distributed inductance and capacitance to perform in the functional capacity as a low pass common mode electronic current filter.
U.S. Pat. No. 4,800,348 discloses an adjustable electronic filter including dielectric block having one or more through holes.
For applications using both existing and new motors, an emerging need exists for reliable methods to control noise, especially RFI noise, both where generated and where received.
Significant opportunity lies not only in new replacement devices meeting updated electrical-related interference noise specifications but also in modular in-line retrofit and/or optional modular in-line device additions to existing and even new systems rendering improved interface and/or optional functional capabilities.
FIGS. 1 and 2 show respective partial radial and termination-end axial views of a typical small motor not incorporating integral electrical noise filtration methods. High volume and low cost production motor end caps are typically composed primarily of plastic with some metallic components, although some motor designs have a nearly complete exterior of metal. A typical motor end cap contains a DC commutator brush assembly.
FIG. 3 shows a cutaway partial radial view of a typical small motor terminal end incorporating a connector.